Extended reality (XR) is a term referring to all real-and-virtual combined environments and human-machine interactions generated by computer technology and wearables, where the 'X' represents a variable for any current or future spatial computing technologies.  e.g. It includes representative forms such as augmented reality (AR), mixed reality (MR) and virtual reality (VR)  and the areas interpolated among them. The levels of virtuality range from partially sensory inputs to immersive virtuality, also called VR.
XR is a superset which includes the entire spectrum from "the complete real" to "the complete virtual" in the concept of reality–virtuality continuum introduced by Paul Milgram. Still, its connotation lies in the extension of human experiences especially relating to the senses of existence (represented by VR) and the acquisition of cognition (represented by AR). With the continuous development in human–computer interactions, this connotation is still evolving.
XR is a rapid growing field being applied in a wide range of ways, such as entertainment, marketing, real-estate, training and remote work.